1. Field of the Invention
The present invention relates to magnet-embedded motors, in which a magnet is embedded in a rotor, used as synchronous motors or brushless motors.
2. Background Art
In conventional magnet-embedded motors, adhesive is applied to a permanent magnet, which is then inserted into a rotor core, for attaching the permanent magnet inside the rotor core. One example of the prior art is disclosed in Japanese Patent Unexamined Publication No. H11-191939.
However, the prior art has a drawback of an increased number of steps in design and production due to the need to control the moisture-resistance and viscosity of the adhesive. In addition, the use of adhesives increases the cost. Furthermore, since a slight clearance is required between the permanent magnet and the permanent magnet embedding hole to carry the adhesive, valid magnetic flux generated at the rotor core is reduced, resulting in reduced torque.
To solve the above disadvantages, the development of a magnet-embedded motor in which the permanent magnet is fixed inside the rotor core without using adhesive has been studied.
FIGS. 5A and 5B and FIG. 6 are magnified views of the permanent magnet and the permanent magnet embedding hole in the rotor core in a conventional magnet-embedded motor.
In FIG. 5A, permanent magnet embedding hole 112 has a minimum clearance with embedded permanent magnet 114. Clearance 115 at both ends in the longer direction is set at the minimum required clearance. Accordingly, permanent magnet 114 can be held inside permanent magnet embedding hole 112 without using adhesive. However, since permanent magnet 114 is magnetized as shown in FIG. 5A, short-circuiting magnetic flux 120 increases. This reduces valid magnetic flux passing to the stator, and thus reduces torque.
In FIG. 5B, permanent magnet embedding hole 113 has a minimum clearance in the shorter direction with embedded permanent magnet 114, and a larger clearance in the longer direction. In other words, permanent magnet embedding hole 113 has a larger clearance 115 at both ends of its long side. This reduces flux short-circuit compared to the case shown in FIG. 5A. However, permanent magnet 114 will move inside permanent magnet embedding hole 113 if adhesive is not used. Accordingly, permanent magnet 114 hits an inner side of permanent magnet embedding hole 113 when the motor is accelerated or decelerated. This may crack or chip the permanent magnet, and also generate noise.
Accordingly, the shape of permanent magnet embedding hole 212 shown in FIG. 6 has been proposed. This permanent magnet embedding hole 212 has protrusion 215 outward from both ends of the long side. This protrusion has a shorter width than the short side of permanent magnet 214. Both ends of the long side to the inner periphery side and protrusion 215 are connected by arc-shaped fillet 217. When rectangular parallelepiped permanent magnet 214 is inserted into permanent magnet embedding hole 212 with this shape, a tip of permanent magnet 214 to the inner periphery side contacts fillet 217. This enables suppression of any movement of permanent magnet 214 during acceleration and deceleration of the motor. At the same time, flux short-circuit, as the case shown in FIG. 5A, can be reduced.
However, with this shape of permanent magnet embedding hole 212, an end of permanent magnet 214 makes a line contact with the inner face of permanent magnet embedding hole 212, and thus the grip on permanent magnet 214 is insufficient. If sudden acceleration or deceleration of the motor takes place, movement of permanent magnet 214 inside permanent magnet embedding hole 212 cannot be completely suppressed.